The present invention relates generally to communication systems. The present invention relates more particularly to a cable modem system wherein information is communicated between a plurality of cable modems and a cable modem termination system.
The desired solution for high speed data communications appears to be cable modem. Cable modem is capable of providing data rates as high as 56 Mbps, and is thus suitable for high speed file transfer, video teleconferencing and pay-per-view television. Further, cable modems may simultaneously provide high speed Internet access, digital television (such as pay-per-view) and digital telephony.
Although cable modems are used in a shared access system, wherein a plurality of subscribers compete for bandwidth over a common coaxial cable, any undesirable reduction in actual data rate is easily controlled simply by limiting the number of shared users on each system. In this manner, each user is assured of a sufficient data rate to provide uninterrupted video teleconferencing or pay-per-view television, for example.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art.
According to an aspect of the invention a modem communicates with a cable transmission system. The modem requests an amount of bandwidth on the cable system to transmit data. The modem receives a grant of an amount of bandwidth to transmit data in response to the request. The modem compares the requested amount of bandwidth with the granted amount of bandwidth. Responsive to the comparison, the modem fragments the data to be transmitted into a plurality of segments if the requested amount is larger than the granted amount. One of the segments is no larger than the granted amount. Responsive to the grant, the modem transmits all the data to be transmitted to the cable system if the requested amount is no larger than the granted amount and transmits only the one segment to the cable system if the requested amount is larger than the granted amount.
In another aspect, the present invention includes a method for facilitating communications between a plurality of cable modems and a cable modem termination system. The method comprises transmitting a request from the cable modem to the cable modem termination system. The request is a request to transmit a data packet from the cable modem to the cable. modem termination system. A time slot for transmitting the data packet from the cable modem to the cable modem termination system is allocated, optionally by the cable modem termination system. Information representative of the time slot is then transmitted, optionally by the cable modem termination system, to the cable modem. The cable modem then transmits the data packet from the cable modem to the cable modem termination system within the allocated time slot.
Transmitting the data packet from the cable modem to the cable modem termination system within the time slot mitigates collisions between data packets which are transmitted by different cable modems to the cable modem termination system at the same time and upon the same frequency channel.
In another aspect, the present invention includes an apparatus and method for synchronizing upstream communications between a plurality of cable modems and a cable modem termination system (CMTS). A first message representative of a frequency of a clock of the cable modem termination system is generated, particularly by the cable modem termination system. Similarly, a second message representative of a slot timing offset of a selected one of the cable modems with respect to the clock of the cable modem termination system is generated, again particularly by the cable modem termination system. The first and second messages are transmitted to the selected cable modem. Frequency synchronization of the clock of the selected cable modem is enhanced with respect to the cable modem termination system utilizing the first message. The slot timing offset of the clock of the cable modem with respect to the clock of the cable modem termination system is compensated for using the second message. This process is repeated, as necessary, for each cable modem which is to communicate with the cable modem termination system.
In another aspect, the present invention includes an apparatus and method for rapidly acquiring data packets in a receiver such as that of a cable modem termination system. The method generally comprises determining fractional symbol timing correction by a feedback loop process, determining carrier phase correction by a feedback loop process and determining fine amplitude by a conventional coherent estimation process.
Optionally, the amplitude of the input to the fractional symbol timing correction circuit and/or the input gain to the carrier phase correction circuit may be adjusted so as to be within the optimal operational range of the fractional symbol timing correction circuit and/or the carrier phase correction circuit via the use of phase detector gain boosting logic, wherein a censor senses the amplitude of the signal input to the fractional symbol timing correction circuit and/or the carrier phase correction circuit and the amplitude of the signal is then modified so as to be within the optimal range.
Optionally, timing recovery may be accelerated by utilizing two out-of-phase, e.g., 180 degree out-of-phase, sampling timing signals such that one of the two sampling timing signals will always be located sufficiently distant from a null point in a binary input sequence as to enhance timing recovery.
Another aspect of the present invention is a method for communicating modulated information from a plurality of cable modems to a cable modem termination system in a way that enhances the robustness of the upstream channels. One way is to monitor upstream channels for at least one modulation parameter which is indicative of channel quality and adjusting the modulation in response to the channel quality parameter.
Another way is to vary the symbol rate of information transmitted over a channel in a near-continuous manner by varying at least one of the upper and lower bounds.
Another way is to monitor the upstream channels and to use fine carrier frequency agility and/or variable symbol rate to facilitate dynamic channel allocation so as to communicate only within channels having quality which is above a predetermined threshold.
According to another aspect of the present invention, slot timing information and/or data-type information is transmitted from a MAC to a burst receiver so as to facilitate processing of upstream data packets by the burst receiver in an efficient and reliable manner. Channel status/error information is transmitted from the burst receiver to the MAC. The error information is used by the MAC to facilitate spectrum management which enhances the data rate and/or reliability of upstream communications.
In an aspect of the invention, noise is abated in a cable system. The cable system has a headend and a plurality of user terminals Interconnected by a cable that has a single downstream transmission path from the headend to each of the user terminals and a plurality of upstream transmission paths from each of the user terminals to the headend. A notch filter at the headend is adjusted to reject one or more bands of common noise. A feedback equalizer (FBE) at the headend compensates for the distortion caused by the notch filter. Afterwards, a ranging signal is received at the headend from each user terminal and feedforward equalizer coefficients corresponding to the transmit equalizer coefficients for each user terminal are generated from the ranging signals. The feedforward equalizer (FFE) coefficients are transmitted to the respective user terminals, where they are applied to a transmit equalizer filter to compensate for individual noise at the respective user terminals.